The present invention relates to load measuring systems, and more particularly, to a system which employs strain gauges on truck axles to measure the load carried by the truck.
In order to maximize profits, drivers of trucks hauling loads on weight regulated highways try to load their trucks so that they are carrying as much as legally permissible. If a truck is overloaded, the driver runs the risk of being fined. The reason that such fines are imposed is that overloaded trucks are a safety hazard and damage the highways.
In order to enable drivers to weigh the loads on their trucks at remote locations without using large platform scales, on-board vehicle weighing systems have been developed. U.S. Pat. No. 3,854,540 of Helmstrom, Jr. discloses a weighing system including strain gauges and transducers for measuring the weight carried by the fifth wheel of a tractor trailer. U.S. Pat. No. 4,020,911 of English, et al. disclose another arrangement of two compression sensitive strain gauges and two tension sensitive strain gauges connected in a Wheatstone bridge circuit on the beam of a truck chassis or trailer chassis. U.S. Pat. No. 4,042,049 of Reichow, et al. discloses a load measuring system for a tandem-wheel tractor truck in which deflections of the load equalizing beams and a front axle are sensed by a load cell transducer mounted on each beam and on the axle. A load indicator provides a visual indication of front and rear wheel axle loading with respect to a preset maximum load point. U.S. Pat. No. 4,102,031 of Reichow, et al is based on a division of the same application that resulted in the aforementioned U.S. Pat. No. 4,042,049. It claims the method of installing a transducer on a structural member. A spacer is detachably secured to a pair of mounting pads and the pads are welded to the structural member. The spacer is then detached and the transducer is attached to the pads. U.S. Pat. No. 4,215,754 of Hagedorn, et al. discloses a load measuring system for leaf spring suspensions. Four spaced apart strain gauges are mounted on the upper surface of the spring beneath the axle clamp, equi-distant from the center of the spring.
Prior on-board load measuring systems for trucks have located the strain gage sensors in the high stress areas of the load beam cap in order to produce the highest output from the sensors. This can lead to premature failure of the beam. Furthermore, installation of the strain gage sensors is tedious and they cannot be readily repaired or replaced. As shown in the aforementioned U.S. Pat. No. 4,102,031 of Reichow, et al, it has been conventional to bolt the sensors to metal mounting pads which are welded to the beam cap. The heat transfer coefficient of the pads has been significantly different than that of the load beam which has contributed to inaccuracy in the measured weight. Also, prior on-board vehicle load measuring systems have generally required field calibration utilizing known weight changes.